1. Technical Field
The present invention relates to a light receiving detection circuit, particularly to a light receiving detection circuit used in a photodetector of a light curtain.
2. Related Art
The light curtain is used as a safety device for an industrial machine and the like (for example, see OMRON corporation product information [online] Jan. 6, 2009, the Internet, <http://www.fa.omron.co.jp/product/family/1581/index_p.html>). FIG. 1 is a schematic diagram illustrating a configuration of a conventional light curtain. A light curtain 11 includes a projector 13 and a photodetector 15. The projector 13 includes plural projection elements 12 each of which includes a light emitting diode (LED). The photodetector 15 includes plural light receiving circuits 14, and a light receiving element is incorporated in each light receiving circuit 14. In the projector 13, the plural projection elements 12 are arrayed in line. In the photodetector 15, the plural light receiving elements 14 are arrayed in line such that the light receiving element 14 is paired with the projection element 12, a switch circuit 16 is connected to each light receiving circuit 14, output terminals of the switch circuits 16 are connected to one another, and the output terminals are connected to an input terminal of a high-pass filter circuit 17, and a signal passing through the high-pass filter circuit 17 is transmitted to a control circuit 18.
The projector 13 and the photodetector 15 are disposed while facing each other with a detection area interposed therebetween. Each projection element 12 emits a light beam toward the corresponding light receiving circuit 14, whereby a curtain or blind for detecting invasion is produced by the plural light beams. However, the projection elements 12 do not always emit the light beam, but the projection elements 12 circularly emit pulsed light beams in order at constant timing (hereinafter the pulsed light beam is referred to as light pulse P).
In the light curtain 11, when a hand or a foreign matter invades in the detection area between the projector 13 and the photodetector 15, the light pulse P passing through the portion is blocked, and the invasion of the hand or the foreign matter is sensed by the photodetector 15.
FIG. 2 is a circuit diagram illustrating a specific circuit configuration of the light receiving circuit 14. The light receiving circuit 14 includes a light receiving element 19 formed by a photodiode (PD), an I/V conversion circuit (current/voltage conversion circuit) 20, and a filter circuit 21. The I/V conversion circuit 20 includes an operational amplifier 23 to which negative feedback is applied by a resistor 22, and a reference power supply 24 is connected between a noninverting input terminal of the operational amplifier 23 and the ground. The light receiving element 19 is connected between an inverting input terminal of the operational amplifier 23 and the ground. The filter circuit 21 includes an amplifier 25, a capacitor 26, a resistor 27, and a DC power supply 28. The capacitor 26 is connected between an input terminal of the amplifier 25 and an output terminal of the I/V conversion circuit 20. The resistor 27 and the DC power supply 28 are connected in series, and the resistor 27 and the DC power supply 28 are also connected between an input of the amplifier 25 and the ground. A high-pass filter that cuts a low frequency component is formed by the capacitor 26 and resistor 27 that are connected into a tau-shape.
In the light receiving circuit 14, a current signal is passes through the light receiving element 19 when the light receiving element 19 receives the light pulse P, the I/V conversion circuit 20 converts the current signal into a voltage signal, stationary light and ambient light are cut by the filter circuit 21 including the capacitor 26 and resistor 27, and the voltage signal is amplified and outputted by the I/V conversion circuit 20 and the amplifier 25. The output signals of the light receiving circuits 14 that face the projection elements 12 emitting the pulsed light is selected by the switch circuit 16, and the selected output signal is fed into the control circuit 18 through the high-pass filter circuit 17.
In a light receiving circuit of the conventional light curtain 11, (1) a time (stability time) until the light receiving signal returns to an initial voltage is long, (2) a variation in gain is large, and (3) a variation in offset voltage outputted from the amplifier is large.
(Stability Time)
First the stability time until the light receiving signal returns to the initial voltage will be described. FIG. 3A illustrates the rectangular light pulse P, FIGS. 3B and 3C illustrate the light receiving signal that is outputted from the amplifier 25 in receiving the light pulse P of FIG. 3A. FIG. 3B illustrates a theoretical waveform of the light receiving signal, and FIG. 3C illustrates an actual waveform of the light receiving signal in consideration of a characteristic of the light receiving circuit 14. In the theoretical waveform, the steep rise and fall of the light receiving signal are exhibited as illustrated in FIG. 3B. On the other hand, in the actual waveform, the light receiving signal dulls and changes gently as illustrated in FIG. 3C, and the light receiving signal return gradually to the initial voltage while undershot after the light pulse P is turned off. At this point, the time until the light receiving signal returns to the initial voltage since the light pulse P is turned off is called stability time.
In the light curtain 11, the light pulse P has a pulse width of about 2.5 μsec on the projector side, and the light pulse P has a repetition period of about 30 μsec. When the light pulse P emitted from a certain projection element 12 is incident to the next-stage light receiving element 19 as illustrated in FIG. 1, the next-stage light receiving element 19 continuously receives the light pulses P in a short time interval as illustrated in FIG. 4. At this point, when the light receiving circuit 14 has the long stability time, in the light receiving element 19 and I/V conversion circuit 20 of the next stage, a signal derived from the light pulse P leaking out from the previous stage and a signal derived from the light pulse P from the corresponding projection element 12 overlap each other. As a result, the light receiving signal includes an error component, and normal detection cannot be performed in the light curtain 11.
In order to avoid the overlap of the light receiving signals, one may lengthen a repetition period of the light pulse P. However, when the repetition period of the light pulse P is lengthened, a proportion of the time the projector 13 does not emit the light pulse P increases to degrade detection accuracy of the light curtain 11. Therefore, in the conventional light curtain 11, the repetition frequency of the light pulse P is set to about 30 μsec, and a need for suppressing the stability time to 20 μsec or less arises.
(Variation in Gain)
In order to shorten the stability time as desired, it is necessary to raise a cutoff frequency of the filter circuit 21. In such cases, as illustrated in FIG. 3B, it is necessary to steeply outputted the pulsed light receiving signal. However, actually the response speed of the amplifier 25 does not follow the light receiving signal, and the light receiving signal becomes the dull-edged waveform as illustrated in FIG. 3C. The dull-edged waveform is influence by a circuit current value or a parasitic capacitance value. Therefore, in a semiconductor integrated circuit, a variation in gain increases in the light receiving circuit 14.
The control circuit 18 compares the fed light receiving signal to a threshold to determine whether or not the invasion is sensed. The threshold is determined based on the light receiving signal fed into the control circuit 18. Therefore, because the threshold becomes unstable when the gains of the I/V conversion circuits 20 vary, it is necessary to reduce the variation in gain of the light receiving circuit 14 in order to enhance the detection accuracy of the light curtain 11.
(Variation in Offset)
In the light receiving circuit 14 of FIG. 2, because the gain of the amplifier 25 is set to 100 times, an input offset voltage of the amplifier 25 is multiplied by 100, and the output offset voltage is supplied from the amplifier 25. For example, when the input offset voltage is ±3 mV, the output offset voltage becomes ±300 mV. As a result, the DC voltage of the initial voltage supplied from the amplifier 25 varies largely. An input level that can be amplified by the amplifier 25 decreases when the output DC voltage varies toward the positive side. Accordingly, it is necessary to suppress the variation in offset voltage supplied from the amplifier 25 as small as possible.